What is the poem "Buffalo Bill's" saying about death? What most suggests the speaker's attitude toward it?

You need a little bit of background, an eye toward the layout of the poem on the page, and a ear for sound to properly interpret Cummings poem.

First of all, Buffalo Bill Cody was once a frontiersman and an "Indian Killer."  A whiz with a gun, after the "Indian Problem" was extinguished, the spotlight-loving Cody became a huge draw to the burgeoning theaterical circus known as "Wild West Shows."  Cody amazed crowds with his trickery and marksmanship.  He, along with the legendary Indian Chief Sitting Bull, and femme-phemon Annie Okley, drew enormous crowds wherever the played. However, the appeal of the Wild West show, like the frontier itself, began to wane and eventually disappeared altogether. 

Turn the poem sideways and you will see that it is shaped like a pistol.   The line "onetwothreefourfive pigeonsjustlikethat" is meant to replicate the sound of a bullet being fired from a gun.  "Jesus," the single word of line seven, is the bullet hitting its mark. 

As for the speaker's feelings about death, keep an eye to the shorter lines:  defunct,  stallion, Jesus, Mister Death.

All life, no matter how shining, will pass.  Cody lived through various incarnations of the self, was a "stallion", was then put out to pasture, and finally claimed, as we all are, by death.